They were shipped back to France to continue fighting, but then the armistice was declared. Anyway it was not that easy to be later a "free french" as it means becoming an outlaw from the army, with possible repercussions on family in France in uncertain times.
– NikkoJan 26 '15 at 9:33

@EvilWashingMachine It made sense at the time. The French surrender was a bit of a surprise. Britain and France had agreed not to make any separate peace. A week after Dunkirk (June 11th at the latest), Paris hadn't yet fallen (it was declared open), the French still had a substantial military and could have defended the south of France at the Loire River. Britain didn't even have the material to equip its own troops, let alone the French, so it made sense to send them back to fight as quickly as possible.
– SchwernJan 27 '15 at 18:49

3

It is pertinent to note that 3000 French soldiers who did not elect to return to France formed the embryo of the Free French forces under de Gaulle.
– AnarylJan 27 '15 at 23:37

More than 100,000 evacuated French troops were moved to camps in various parts of south-western England, where they were temporarily lodged before being repatriated. British ships ferried French troops to Brest, Cherbourg, and other ports in Normandy and Brittany, although only about half of the repatriated troops were deployed against the Germans before the surrender of France. For many French soldiers, the Dunkirk evacuation represented only a few weeks' delay before being killed or captured by the German army after their return to France. Of the French soldiers evacuated from France in June 1940, about 3,000 joined Charles de Gaulle's Free French army in Britain. At least one ship repatriating the French soldiers to France was sunk by the Germans, with great loss of life.